This invention relates to improved gas diffusion electrodes for use in primary batteries, fuel cells and in electrochemical cells such as those used in alkali metal halide electrolysis processes.
In the above applications, the electrodes have different size and current density requirements but research and development carried out with respect to these types of electrodes generally seeks to either increase the operating current density for a given voltage savings or to increase voltage savings at a given current density.
Structurally, gas diffusion electrodes used as cathodes or anodes are layered composites comprising an electrochemically active layer and a wetproofing layer. In at least one major method of manufacturing, these electrodes have been made by first pressing separately an active layer containing active material, a fluorocarbon polymer such as polytetrafluoroethylene and a pore-forming salt, and a wetproofing layer containing the fluorocarbon polymer and the salt. The two preformed layers are then pressed together. A metallic screen current collector may also be embedded into the active layer to form a three-layered structure.
Problems have developed with electrodes produced by this method. Often, after a short time in service, delamination or disbonding occurs between the active and wet proofing layers with electrodes made in this manner. Bubbles are seen to form in the wetproofing layer which appears to expand. When this structural problem occurs, the cell ceases to operate and this can disrupt performance of multi-unit, stacked cell assemblies.
Other methods for producing gas diffusion electrodes also have been reported.
U.S. Pat. No. 3,385,780 discloses the production of a porous electrode by placing a mixture of an electrically conductive substance, a hydrophobic polymer, and a removable filler in a press, placing thereover a mixture of the hydrophobic polymer and the removable filler, pressing the two mixtures together, and then removing the filler material.
German Patent Application No. 2,111,095 describes a method of producing electrodes by separately filtering wet suspensions of active and wetproofing material in layers to form "filter cakes". The active layer may contain a conductive grid. The filter cake layers are then stacked, pressed together, and sintered to form an electrode. The German Application further discloses that an electrode can be produced by filtering an active suspension through the pores of a wetproofing layer filter cake and then pressing and sintering the filter cake.
U.S. Pat. No. 3,594,236 discloses a method of producing an air electrode by first pressing a sheet containing a fluorocarbon polymer and a pore forming agent and thereafter applying a layer of a catalyst and a metallic grid on the fluorocarbon polymer sheet and hot pressing the assembly to form a unitary electrode structure.